A wireless network generally comprises of many smaller cells to cover the whole service area. Each cell is further divided into multiple sectors. Each cell/sector may have a base station (BS) and multiple mobile stations (MSs). Cellular system with 3-sectors per cell is depicted in FIG. 1. The MSs in a sector may be fixed, nomadic or mobile. Communication from a BS to an MS is called as downlink or forward link. Similarly, communication from an MS to a BS is called as uplink or reverse link. In IEEE 802.16m system a BS is denoted as advanced BS (ABS) and an MS is denoted as advanced MS (AMS). Similarly in LTE/LTE-Advanced a BS is denoted as e-NodeB and a MS is denoted as UE.
The IEEE 802.16m, LTE and LTE-Advanced are broadband wireless standards that use Orthogonal Frequency Division Multiplexing Access (OFDMA) technology in the downlink. The block diagram of an OFDMA based system is shown in FIG. 2. The IEEE 802.16m uses OFDMA, and the LTE/LTE-Advanced use DFT spread OFDMA (a.k.a. SCFDMA) technology in the uplink.
In the IEEE 802.16m, LTE and LTE-Advanced standards, resources are allocated in a time-frequency grid called a resource block (RB) or physical resource unit (PRU) that consists of P subcarriers and Q OFDM symbols or multiples of P subcarriers and Q OFDM symbols. The value of P and Q can be any integer, and the value of P and Q are dependent on the individual standards. The P subcarriers can be physically contiguous or distributed, and in case of distributed, permutation can be subcarrier wise or groups of subcarrier wise.
In the downlink, one or more RBs may be intended for single or group of users; in the uplink, a transmitter may be assigned one or more RBs and several transmitters may transmit simultaneously. The PSK/QAM input data are mapped to distinct subcarriers, and filled with zeros in the unused subcarriers before taking an N-point IDFT.
When the P subcarriers are adjacent, it is possible to do Channel Dependent multi-user Scheduling (CDS) and improve the throughput of the system. In CDS, users requesting resources with good channel quality are given preference in scheduling. In distributed modes, the P subcarriers are distributed over the entire available bandwidth (for instance, in a pseudo-random fashion that can include fast hopping across the tones) and interference from adjacent tones is averaged and frequency diversity is exploited inherently.
The localized resource unit, also known as Contiguous Resource Unit (CRU) contains a group of subcarriers which are contiguous across the localized resource allocations. The minimum size of the CRU equals the size of the PRU, i.e., P subcarriers by Q OFDMA symbols. The resource allocated to a user or a group of users will be in multiples of the basic resource units, and it can be either contiguous or distributed. N1 contiguous basic resource units are called as sub-band, and N2 contiguous resource units are called as mini-band in IEEE 802.16m standards. N1 and N2 are positive integers. Typical number for N1 is 3, 4 or 5 and N2 is 1 or 2. The miniband CRUs available in a frequency partition can be divided into two groups. The first group can be used as miniband CRU itself, and the second group will be used to create subcarrier, or pairs of subcarrier, groups of subcarrier (tile) permuted distributed resource unit (DRU).
In the IEEE 802.16m systems, the total available physical resource is divided into logical resources to support scalability, multiple accesses. The logical resources are called as Logical Resource Units (LRU), and each LRU is composed of 18 contiguous CRU or pair-wise subcarrier permutation over the entire available bandwidth (DRU) and Q contiguous OFDM symbols. When LRU is composed of CRU, each LRU is further divided into miniband CRU (NLRU) with N2=1 and consisting of 18 contiguous subcarriers and subband CRU (SLRU) with N1=4 and consisting of 72 contiguous subcarriers. When the DRU is derived from NLRU, the LRU is called as Distributed Logical Resource Unit (DLRU), and the LRU consists of 18 subcarriers. In IEEE 80216m systems, the DLRU contains a group of paired subcarriers spread across the distributed resources within a frequency partition. The minimum unit for forming the DLRU is equal to a pair of subcarriers, called tone-pair.
FIGS. 4 and 5 illustrate examples of RB or PRU structure used in downlink of the IEEE 802.16m standards. In every PRU, certain subcarriers are reserved for pilot tones and the pilot tones used for estimating the channel between the transmitter and receiver. In OFDMA systems, the localized and distributed sub-channelization methods provide a great flexibility in reaping the benefits of both single user and multi-user diversity.
The advanced air interface basic frame structure is illustrated in FIG. 3. Each 20 ms superframe is divided into four equally-sized 5 ms radio frames. When using the channel bandwidth of 5 MHz, 10 MHz, or 20 MHz, each 5 ms radio frame further consists of eight Advanced Air Interface (AAI) subframes for G=⅛ and 1/16. For G=¼, the 5 ms radio frame consists of seven AAI subframes. With the channel bandwidth of 8.75 MHz, the 5 ms radio frame consists of seven AAI subframes for G=⅛ and 1/16, and six AAI subframes for G=¼. With the channel bandwidth of 7 MHz, the 5 ms radio frame consists of six AAI subframes for G= 1/16, and five AAI subframes for G=⅛ and G=¼. An AAI subframe shall be assigned for either DL or UL transmission.
There are four types of AAI subframes:
1) type-1 AAI subframe which consists of six OFDMA symbols,
2) type-2 AAI subframe which consists of seven OFDMA symbols,
3) type-3 AAI subframe which consists of five OFDMA symbols, and
4) type-4 AAI subframe which consists of nine OFDMA symbols. This type shall be applied only to an UL AAI subframe for the 8.75 MHz channel bandwidth when supporting the Wireless MAN-OFDMA frames. The size of Q depends on the AAI subframe types as mentioned above.
The basic frame structure is applied to FDD and TDD duplexing schemes, including H-FDD AMS operation. The number of switching points in each radio frame in TDD systems shall be two, where a switching point is defined as a change of directionality, i.e., from DL to UL or from UL to DL. When H-FDD AMSs are included in an FDD system, the frame structure from the point of view of the H-FDD AMS is similar to the TDD frame structure. However, the DL and UL transmissions occur in two separate frequency bands. The transmission gaps between DL and UL and between UL and DL are required to allow switching the TX and RX circuitry.
A data burst shall occupy either one AAI subframe (i.e. the default TTI transmission) or contiguous multiple AAI subframes (i.e. the long TTI transmission). Any 2 long TTI bursts allocated to an AMS shall not be partially overlapped, i.e. any 2 long TTI bursts in FDD shall either be over the same 4 subframes or without any overlap. The long TTI in FDD shall be 4 AAI subframes for both DL and UL. For DL (UL), the long TTI in TDD shall be all DL (UL) AAI subframes in a frame.
The transmission of predefined (known) sequences on the pilot subcarriers in the downlink is necessary for enabling channel estimation, measurements of channel quality indicators (CQI) such as the SINR, frequency offset estimation, etc. To optimize the system performance in different propagation environments and applications, AAI of IEEE 802.16m supports both common and dedicated pilot structures. The categorization in common and dedicated pilots is done with respect to the usage of common and dedicated pilots. The common pilots can be used by all MSs and the pilots are precoded in the same way as the data subcarriers within the same PRU. Dedicated pilots can be used with both localized and distributed allocations. The dedicated pilots are associated with a specific resource allocation and are intended to be used by the MSs allocated to said specific resource allocation. Therefore dedicated pilots shall be precoded or beamformed in the same way as the data subcarriers of the resource allocation. The pilot structure is defined for up to eight transmission (Tx) streams and there is a unified pilot pattern design for common and dedicated pilots. There is equal pilot density per Tx stream, while there is not necessarily equal pilot density per OFDMA symbol of the downlink AAI subframe. Further, within the same AAI subframe there is equal number of pilots for each PRU of a data burst assigned to one MS. Pilot patterns are specified within a PRU. The base pilot patterns used for two DL data streams in dedicated and common pilot scenarios are shown in FIG. 4, with the subcarrier index increasing from top to bottom and the OFDM symbol index increasing from left to right. Subfigure (a) and Subfigure (b) in FIG. 4 shows the pilot location for pilot stream 1 and pilot stream 2 in a PRU, respectively. The number on a pilot subcarrier indicates the pilot stream the pilot subcarrier corresponds to. The subcarriers marked as ‘X’ are null sub-carriers, on which no pilot or data is transmitted. The interlaced pilot patterns are generated by cyclic shifting of the base pilot patterns. The interlaced pilot patterns are used by different BSs for one and two streams. Interlaced pilot patterns for one stream is shown in FIG. 5 and interlaced pilot patterns on stream 1 and stream 2 for two streams are shown in FIG. 6 and FIG. 7, respectively. Each BS chooses one of the three pilot pattern sets (pilot pattern set 0, 1, and 2) as shown in FIG. 5, FIG. 6 and FIG. 7. The index of the pilot pattern set used by a particular BS with Cell ID=k is denoted by pk. The index of the pilot pattern set is determined by the Cell ID according to the following equation: pk=floor (k/256).
For one stream, each ABS additionally chooses one of the two stream sets (stream set 0 and 1) within each pilot pattern set. The index of the stream, denoted by sk, shall be determined according to the following equation: sk=mod (k, 2). For the AAI subframe consisting of 5 symbols, the last OFDM symbol in each pilot pattern set shown in FIG. 4 is deleted. For the AAI subframe consisting of 7 symbols, the first OFDM symbol in each pilot pattern set shown in FIG. 4 is added as 7th symbol.
Communication between the BS and the MS and vice-versa requires spectrum. Spectrum is a very scarce resource, and the spectrum further limited due to pre-occupation of some portion of the bands for other applications such as defense and space in some countries. The available spectrum will be reused in every cells/sectors. Since same frequency band (bandwidth) is reused in different cells/sectors depending on the reuse factor, the subscriber at the boundary between regions will be severely affected by interference. This phenomenon is called as co-channel interference (CCI), and the performance for the subscriber in these cell edge regions is severely affected by the CCI. This predominantly limits the cell edge throughput, and hence brings down the overall system throughput. The problem is even worse in the case of the emerging broadband wireless technologies such as IEEE 802.16m, LTE and LTE-Advanced, where the available frequency resource is expected to be used in a frequency reuse 1 fashion in every sector in order to meet the high data rate requirements of the subscribers. Therefore, the major challenge in developing the above mentioned emerging broadband wireless technologies is to mitigate interference.
Interference can be mitigated using simple receiver processing techniques like interference suppression minimum mean square error (MMSE) receivers. There is another interference mitigation technique called conjugate data repetition (CDR), where a transmission scheme repeats data in a predefined fashion across cells/sectors relying on multiple copies of the transmitted signal, multiple receive antennas, and MMSE receivers. The techniques can be employed to suppress interference, and thereby improve the reliability as well as throughput for cell edge users.
One of the major challenges in the design of interference suppression MMSE receiver is to obtain a good quality estimation of the desired fading channel and the ‘interference plus noise’ covariance matrix. The reference signals or pilots are transmitted by the base station (BS) or by the MS for the purpose of channel estimation, and also for the interference plus noise covariance estimation. In the interference limited scenario, because of the frequency reuse, these reference signal or pilots will also be affected by severe CCI. This in turn affects the quality of channel estimates and interference covariance estimates, which in turn affect the throughput of the cell edge users.
Consider a cellular layout with 3 sectors cells as shown in FIG. 1. In general, the strongest interference for a cell edge user comes from those sectors with sector numbers different from its desired one. For example in FIG. 1, the user with sector number 0 receives the strongest interference from those surrounding sectors with sector numbers 1 and 2.
The SINR seen by the pilot symbols can be improved by avoiding pilot to pilot collisions between sectors with different sector numbers using interlaced pilots. Each sector number is assigned a pilot pattern, in a set of locations in the 2-dimensional frequency-time grid within a PRU, which does not collide with those used by other sectors with different sector number. For example in IEEE802.16m, pilot pattern used by sector 0 is shown in FIG. 4. The Sector 1 and Sector 2 use cyclically shifted version of the pilot pattern used by sector 0 as shown in FIGS. 5, 6 and 7. The SINR seen by the pilot symbols can also be improved by boosting the power of pilot tones with respect to data tones. The pilot tones receive interference from the data tones of the neighboring sectors. The pilot boosting helps to improve the received signal-to-interference-plus-noise-ratio (SINR) of the pilot tones.
The pilot tones are boosted at the expense of data tone's power. The power on data tones has to be reduced to keep the total transmitted power the same. This reduces data SINR and results in higher error rate. The data tones transmitted at the locations corresponding to the pilot positions of the neighboring sectors see heavy interference resulting in data erasures. The interference covariance estimates measured from pilot tones are not accurate since the number of pilots within a PRU is small to get enough averaging. Moreover, the interference covariance of those data tones interfered by pilot tones are different from those interfered by data tones. The interference suppression receivers may not work efficiently with the conventional techniques.
Another aspect affecting the reliability of CQI (for example the post-processing MMSE SINR estimates) measurement is the multi-antenna precoder used at the transmitter. When the desired signal as well as interfering signals employ multiple antennas for transmission, the signal as well as interference measured at the receiver become a function of the multi-antenna precoder employed at the respective transmitters which vary in frequency and time continuously based on the feedback from the respective receivers. In systems employing closed-loop precoded transmission, the CQI changes from time to time. This change in CQI causes the multi-user scheduler to allocate incorrect modulation and coding rate (MCS) and therefore causes degradation in system capacity.
The IEEE 802.16m standard uses open-loop (OL) region in the downlink. The OL region is defined as a time-frequency resource using a given pilot pattern and a given open-loop MIMO mode without closed loop rank adaptation. The open-loop region allows base stations across different cells/sectors to coordinate their open-loop MIMO transmissions, in order to offer a stable interference environment where the precoders and numbers of streams are not time-varying. The resource units used for the open-loop region are indicated in a downlink broadcast message and the resource units shall be aligned across cells.
The DL OL region consists of a rank-1 OL MIMO region where only a single data stream is transmitted across multiple antennas and uses single stream rank-1 OL precoding. The precoder is kept constant for the duration of the resource block (RB) or groups of RBs and the precoder may change from one (or group of) resource block to another. In OL region, the precoder that is used in each RB is pre-defined. Data and pilots in each RB are precoded using the same precoder.